这是否更接近您的预期？
遍历u和n的索引。将两个列表中当前索引处的元素连接起来并追加到列表k。

import re
import requests

t=[]
u = []
n = []
k = []
exclude_banned = True

with open("u.txt", "r") as f:
    h = f.readlines()[0:100]
    for sub in h:
        t.append(re.sub('\n', '', sub))
    print(type(t))
    print(t)
    r = requests.post('https://users.roblox.com/v1/usernames/users', json={'usernames': t, 'excludeBannedUsers': exclude_banned})
    print(r.json())
    y = r.json()['data']
    u.extend([f['id'] for f in y])
    n.extend([f['name'] for f in y])

    # Concatenate elements in u and n using a loop
    for i in range(len(u)):
        k.append(f'{u[i]}:{n[i]}')

    print(k)

import re
import requests

t=[]
u = []
n = []
k = []
exclude_banned = True

with open("u.txt", "r") as f:
    h = f.readlines()[0:100]
    for sub in h:
        t.append(re.sub('\n', '', sub))
    print(type(t))
    print(t)
    r = requests.post('https://users.roblox.com/v1/usernames/users', json={'usernames': t, 'excludeBannedUsers': exclude_banned})
    print(r.json())
    y = r.json()['data']
    u.extend([f['id'] for f in y])
    n.extend([f['name'] for f in y])

    # Concatenate elements in u and n using a loop
    for i in range(len(u)):
        k.append(f'{u[i]}:{n[i]}')

    print(k)

我建议做一些简化。例如，如果你对re所做的只是去掉后面的换行符，那么有一个更好的方法。在那之后，我认为你真的需要考虑列表理解来构建最终的用户数据。
退房：

import requests

## ---------------------------
## Read usernames out of a text file.
## ---------------------------
with open("u.txt", "r") as file_in:
    post_data = {
        "usernames": [username.strip() for username in file_in][:100],
        "excludeBannedUsers": True
    }
## ---------------------------

post_url = "https://users.roblox.com/v1/usernames/users"
api_response = requests.post(post_url, post_data)

if not api_response.ok():
    print("something went wrong")
    exit()

## ---------------------------
## Build a list of dictionaries based on the api data
## ---------------------------
user_data = [
    {user["id"]: user["name"]}
    for user
    in api_response.json()["data"]
]
## ---------------------------

## ---------------------------
## Print the results
## ---------------------------
import json
for user in user_data:
    print(json.dumps(user, indent=4))
## ---------------------------